MgB2　superconducting　materials　were　prepared　with　PIT　(powder　in　tube)　method.　The　magnetic　flux　penetration　depth　(λ＇)　of　the　MgB2　samples　were　measured　in　Campbell　method　at　the　different　additional　alternating　magnetic　flux　density　(bac),　and　the　critical　current　density　in　different　magnetic　fields　were　calculated.　The　error　causations　of　the　bac　vs　λ＇　curves　and　the　principle　that　the　smooth　parts　in　the　curve　are　selected　as　available　segments　to　calculate　were　briefly　discussed.　Characteristics　that　the　integral　microstructure　of　the　sample　influences　the　current　is　taken　into　consideration　in　this　measure　method,　the　critical　current　density　obtained　in　this　way　is　more　correspond　with　reality.